The present invention relates to a rotary head type digital audio signal reproducing apparatus suitable for reproducing digital PCM audio signals that have been recorded in the form of signal helical tracks on a recording medium, one track being formed per unit time, with a rotary head.
A technique is known in which audio signals are recorded on magnetic tape with a helical scanning rotary head in the form of helical tracks, one track being formed per unit time, and reproduced thereafter. A digital signal recording reproducing apparatus known as R-DAT (rotary head type digital audio tape recorder) has been designated for recording audio signals, such as PCM signals, and thereafter reproducing the same.
The format of tracks recorded with an R-DAT has a pattern as shown in FIG. 1A. Each of recording regions SUB and PCM is composed of a plurality of blocks as shown in FIG. 1B. The numerals in FIG. 1A represent the numbers of blocks occupied by the respective regions.
ATF-1 (ATF=automatic track finding) between SUB-1 and PCM and ATF-2 between PCM and SUB-2 are each a region provided for ensuring that tracking control i.e., control for allowing a rotary head to correctly scan the recorded tracks during reproduction, can be accomplished only in response to the output of the head, that is, without the need for employing a special head.
In an R-DAT, the pilot signals recorded in the two ATF regions are such that during playback of magnetic tape, the recorded tracks are scanned with two rotary heads having a scanning width larger than the width of each track, and that the reproduction output of the pilot signals from the two tracks adjacent to the track being scanned is used to control the tracking of the rotary heads. The track pattern for ATF is as shown in FIG. 2. ATF-1 and ATF-2 located in the front and rear portions, respectively, of each track have a low frequency (small azimuth-effect) signal f.sub.1 which is employed as a tracking pilot signal. This signal is used for the purpose of detecting the levels of crosstalk for each of the two tracks adjacent to the track being reproduced. The difference between these levels is employed as a tracking error signal.
In each of ATF-1 and ATF-2 is recorded a sync signal for identifying the location at which the pilot signal f.sub.1 is recorded. In the presence of crosstalk, the sync signal is unable to distinguish the on-track signal from the signals from adjacent tracks. Thus, the sync signal is selected in such a way that it not only has a frequency capable of producing an azimuth-effect but also has a pattern which is unique with respect to the PCM signal. If the head having a + (positive) azimuth is designated A and the head having a -(negative) azimuth as B, two different sync signals are provided for the purpose of distinguishing head A from B. Stated more specifically, a sync 1 signal f.sub.2 having a frequency of f.sub.M /18 (=522 kHz) and a sync 2 signal f.sub.3 having a frequency of f.sub.M /12 (=784 kHz), associated with heads A and B, respectively, are recorded in predetermined positions.
In an R-DAT which does not employ an erasing head, a new signal is written over the previously recorded signal. In order to enable such overwriting, an erase signal f.sub.4 having a frequency of f.sub.M /6 (1.56 MHz) is recorded at a predetermined position for erasing the previously recorded pilot signal f.sub.1, sync 1 signal f.sub.2, and sync 2 signal f.sub.3.
The pilot signals for ATF are located at different positions on the on-track (i.e., the track presently being scanned) and the two adjacent tracks. The level of the pilot signal on the on-track differs on a time basis from the levels of each of the pilot signals on the adjacent tracks, and hence the three different levels can be sampled independently of each other.
Five blocks are assigned to each of the ATF regions ATF-1 and ATF-2, and the pilot signal f.sub.1 is recorded in two of these blocks. The sync signals f.sub.2 and f.sub.3 are recorded in an area covering one or one-half block from the center of the position in which one of the two adjacent tracks is recorded. The pilot signal f.sub.1 on the other adjacent track is recorded in such a way that its center is positioned two blocks after the beginning of the sync signal recorded on the on-track. A sync signal composed of one block is assigned to an odd-numbered fame, and a sync signal composed of a half block is assigned to an even-numbered frame.
As described above, the sync signals to be recorded in the ATF region have different frequencies depending upon which head is used in scanning, and these sync signals also have different recording lengths in odd-numbered frames and even-numbered frames. This design enables four consecutive tracks to be distinguished from one another since they are provided with different ATF regions. The pattern of the ATF regions is of the four-track completed type in which the pattern is cyclically repeated every four tracks.
When a magnetic tape on which signals have been recorded in the format shown in FIG. 1A is played back with a rotary head, an RF signal of the type shown in FIG. 3A is reproduced from the head. If this RF signal is obtained by the playback of a track with an odd-numbered frame (A) as shown in FIG. 2, it may be passed through a bandpass filter (BPF) of 130 kHz so as to obtain a pilot signal f.sub.1 as shown in FIG. 3B.
The signal in zone I is due to the pilot signal on the on-track, and those in zones II and III result from crosstalk of the pilot signals on a track with an odd-numbered frame (B) and on a track with an even-numbered frame (B), respectively. If the rotary head were scanning the on-track correctly, the envelope levels of zones II and III, or the values of V.sub.II and V.sub.III indicated in FIG. 3C would be equal to each other. However, if a tracking deviation occurs, V.sub.II is not equal to V.sub.III (V.sub.II .noteq.V.sub.III ) and the amount and direction of the deviation of the rotary head with respect to the ontrack can be determined by the magnitude and polarity of the difference between V.sub.II and V.sub.III Therefore, by actuating a capstan servo according to the difference between V.sub.II and V.sub.III so as effect fine adjustment of the tape speed, the rotary head can be controlled to stay correctly, centered on the on-track.
In order to achieve the operation described above, it is necessary that the sync signals located at predetermined positions be correctly detected for sampling the levels of V.sub.II and V.sub.III. In practice, however, an R-DAT which does not have an erase head and which performs second and third cycles of recording using an overwrite mode often fails to correctly detect the sync signals and to sample V.sub.II and V.sub.III for generating a correct error signal.
In an R-DAT, signals may be recorded within two blocks on either side of the center of the PCM region. The level of recording of the pilot signal f.sub.1 (130 kHz) in an R-DAT is slightly lower than those of recording other signals. This is done in order to ensure that the previously recorded pilot signal f.sub.1 can be erased by an erase signal f.sub.4 in the overwrite mode since signals having lower frequencies are recorded at deeper levels in the magnetic tape. However, reducing the recording level of the pilot signal f.sub.1 sometimes causes a problem in that the previously recorded sync signal f.sub.2 or f.sub.3 cannot be completely erased if a new pilot signal f.sub.1 is written over these sync signals.
Depending on the algorithm (i.e., the set of processes) employed for sync detection, incomplete erasure of previous sync signals may take place either ahead of or behind newly recorded sync signals but the effect of either phenomenon is the same and must be circumvented. Otherwise, the level of the frequency component of the pilot signal in the currently reproduced RF signal will be sampled according to the previously recorded sync signal. The level of this pilot signal should theoretically be equal to that of the crosstalk of the sampling signal from one of the two tracks adjacent to the on-track. In fact, however, the frequency components sampled is actually the pilot signal from the on-track, and hence an extremely large signal level will be produced as a result of this sampling. Subsequently, the frequency component of the pilot signal in the RF signal being reproduced two blocks later is sampled and the difference between the two sampled values, one being currently obtained and the other obtained two blocks previously, is used as the level of track deviation applied to control the capstan servo. However, the previously sampled level relates to the on-track rather than that of the crosstalk from an adjacent track, and the level difference obtained is of such a great magnitude that it is far from being indicative of the actual amount of track deviation. In this situation, the operation of the capstan servo is disturbed, causing adverse effects on tape transport.
In the overwrite mode where the previously recorded pilot signal f.sub.1 is erased with a new pilot signal f.sub.1 there occurs no problem if the same apparatus is employed which recorded the signals. In practice, however, the recording level various from one apparatus to another, and especially between different manufacturers. In this regard, apparatus A may have, for example, a deep recording level whereas set B affords a shallow recording level. In this situation, there is no problem in using the apparatus A to write new information over information which has been recorded on the tape with the apparatus B. In the opposite case where the apparatus B is used to write information over information which has been recorded with set A, the previously recorded pilot signal f.sub.1 cannot be completely erased and will interfere with the overwritten pilot signal by being added to it or being subtracted from it. Even if the sync signal is correctly detected, the interference by the previously recorded pilot signal precludes detection of the correct amount of track deviation.